# ProofObjectsThe Curry-Howard Correspondence

"

*Algorithms are the computational content of proofs*." —Robert Harper
We have seen that Coq has mechanisms both for
We have already seen the fundamental idea: provability in Coq is
represented by concrete
If the proposition is an implication like A → B, then its proof
will be an evidence
Question: If evidence is data, what are propositions themselves?
Answer: They are types!
Look again at the formal definition of the ev property.

*programming*, using inductive data types like nat or list and functions over these types, and for*proving*properties of these programs, using inductive propositions (like ev), implication, universal quantification, and the like. So far, we have mostly treated these mechanisms as if they were quite separate, and for many purposes this is a good way to think. But we have also seen hints that Coq's programming and proving facilities are closely related. For example, the keyword Inductive is used to declare both data types and propositions, and → is used both to describe the type of functions on data and logical implication. This is not just a syntactic accident! In fact, programs and proofs in Coq are almost the same thing. In this chapter we will study how this works.*evidence*. When we construct the proof of a basic proposition, we are actually building a tree of evidence, which can be thought of as a data structure.*transformer*: a recipe for converting evidence for A into evidence for B. So at a fundamental level, proofs are simply programs that manipulate evidence.Print ev.

(* ==>

Inductive ev : nat -> Prop :=

| ev_0 : ev 0

| ev_SS : forall n, ev n -> ev (S (S n)).

*)

Suppose we introduce an alternative pronunciation of ":".
Instead of "has type," we can say "is a proof of." For example,
the second line in the definition of ev declares that ev_0 : ev
0. Instead of "ev_0 has type ev 0," we can say that "ev_0
is a proof of ev 0."
This pun between types and propositions — between : as "has type"
and : as "is a proof of" or "is evidence for" — is called the

*Curry-Howard correspondence*. It proposes a deep connection between the world of logic and the world of computation:propositions ~ types proofs ~ data valuesMany useful insights follow from this connection. To begin with, it gives us a natural interpretation of the type of the ev_SS constructor:

This can be read "ev_SS is a constructor that takes two
arguments — a number n and evidence for the proposition ev
n — and yields evidence for the proposition ev (S (S n))."
Now let's look again at a previous proof involving ev.

As with ordinary data values and functions, we can use the Print
command to see the

*proof object*that results from this proof script.Print ev_4.

(* ===> ev_4 = ev_SS 2 (ev_SS 0 ev_0)

: ev 4 *)

As a matter of fact, we can also write down this proof object

*directly*, without the need for a separate proof script:
The expression ev_SS 2 (ev_SS 0 ev_0) can be thought of as
instantiating the parameterized constructor ev_SS with the
specific arguments 2 and 0 plus the corresponding proof
objects for its premises ev 2 and ev 0. Alternatively, we can
think of ev_SS as a primitive "evidence constructor" that, when
applied to a particular number, wants to be further applied to
evidence that that number is even; its type,
You may recall (as seen in the Logic chapter) that we can
use function application syntax to instantiate universally
quantified variables in lemmas, as well as to supply evidence for
assumptions that these lemmas impose. For instance,

∀n, ev n → ev (S (S n)),

expresses this functionality, in the same way that the polymorphic
type ∀ X, list X expresses the fact that the constructor
nil can be thought of as a function from types to empty lists
with elements of that type.
We can now see that this feature is a trivial consequence of the
status the Coq grants to proofs and propositions: Lemmas and
hypotheses can be combined in expressions (i.e., proof objects)
according to the same basic rules used for programs in the
language.

# Proof Scripts

*proof objects*we've been discussing lie at the core of how Coq operates. When Coq is following a proof script, what is happening internally is that it is gradually constructing a proof object — a term whose type is the proposition being proved. The tactics between Proof and Qed tell it how to build up a term of the required type. To see this process in action, let's use the Show Proof command to display the current state of the proof tree at various points in the following tactic proof.

Theorem ev_4'' : ev 4.

Proof.

Show Proof.

apply ev_SS.

Show Proof.

apply ev_SS.

Show Proof.

apply ev_0.

Show Proof.

Qed.

At any given moment, Coq has constructed a term with some
"holes" (indicated by ?1, ?2, and so on), and it knows what
type of evidence is needed at each hole.
Each of the holes corresponds to a subgoal, and the proof is
finished when there are no more subgoals. At this point, the
evidence we've built stored in the global context under the name
given in the Theorem command.
Tactic proofs are useful and convenient, but they are not
essential: in principle, we can always construct the required
evidence by hand, as shown above. Then we can use Definition
(rather than Theorem) to give a global name directly to a
piece of evidence.

All these different ways of building the proof lead to exactly the
same evidence being saved in the global environment.

Print ev_4.

(* ===> ev_4 = ev_SS 2 (ev_SS 0 ev_0) : ev 4 *)

Print ev_4'.

(* ===> ev_4' = ev_SS 2 (ev_SS 0 ev_0) : ev 4 *)

Print ev_4''.

(* ===> ev_4'' = ev_SS 2 (ev_SS 0 ev_0) : ev 4 *)

Print ev_4'''.

(* ===> ev_4''' = ev_SS 2 (ev_SS 0 ev_0) : ev 4 *)

Theorem ev_8 : ev 8.

Proof.

(* FILL IN HERE *) Admitted.

Definition ev_8' : ev 8 :=

(* FILL IN HERE *) admit.

☐

# Quantifiers, Implications, Functions

*constructors*introduced by Inductive-ly defined data types, and

*functions*.

Theorem ev_plus4 : ∀n, ev n → ev (4 + n).

Proof.

intros n H. simpl.

apply ev_SS.

apply ev_SS.

apply H.

Qed.

What is the proof object corresponding to ev_plus4?
We're looking for an expression whose

*type*is ∀ n, ev n → ev (4 + n) — that is, a*function*that takes two arguments (one number and a piece of evidence) and returns a piece of evidence! Here it is:Definition ev_plus4' : ∀n, ev n → ev (4 + n) :=

fun (n : nat) ⇒ fun (H : ev n) ⇒

ev_SS (S (S n)) (ev_SS n H).

Check ev_plus4'.

(* ===> ev_plus4' : forall n : nat, ev n -> ev (4 + n) *)

Recall that fun n ⇒ blah means "the function that, given n,
yields blah," and that Coq treats 4 + n and S (S (S (S n)))
as synonyms. Another equivalent way to write this definition is:

Definition ev_plus4'' (n : nat) (H : ev n) : ev (4 + n) :=

ev_SS (S (S n)) (ev_SS n H).

Check ev_plus4''.

(* ===> ev_plus4'' : forall n : nat, ev n -> ev (4 + n) *)

When we view the proposition being proved by ev_plus4 as a
function type, one aspect of it may seem a little unusual. The
second argument's type, ev n, mentions the
Notice that both implication (→) and quantification (∀)
correspond to functions on evidence. In fact, they are really the
same thing: → is just a shorthand for a degenerate use of
∀ where there is no dependency, i.e., no need to give a
name to the type on the LHS of the arrow.
For example, consider this proposition:

*value*of the first argument, n. While such*dependent types*are not found in conventional programming languages, they can be useful in programming too, as recent work in the functional programming community demonstrates.
A proof term inhabiting this proposition would be a function
with two arguments: a number n and some evidence E that n is
even. But the name E for this evidence is not used in the rest
of the statement of ev_plus2, so it's a bit silly to bother
making up a name for it. We could write it like this instead,
using the dummy identifier _ in place of a real name:

Or, equivalently, we can write it in more familiar notation:

In general, "P → Q" is just syntactic sugar for
"∀ (_:P), Q".

# Connectives as Inductive Types

## Conjunction

Notice the similarity with the definition of the prod type,
given in chapter Poly; the only difference is that prod takes
Type arguments, whereas and takes Prop arguments.

Print prod.

(* ===>

Inductive prod (X Y : Type) : Type :=

| pair : X -> Y -> X * Y. *)

This should clarify why destruct and intros patterns can be
used on a conjunctive hypothesis. Case analysis allows us to
consider all possible ways in which P ∧ Q was proved — here
just one (the conj constructor). Similarly, the split tactic
actually works for any inductively defined proposition with only
one constructor. In particular, it works for and:

Lemma and_comm : ∀P Q : Prop, P ∧ Q ↔ Q ∧ P.

Proof.

intros P Q. split.

- intros [HP HQ]. split.

+ apply HQ.

+ apply HP.

- intros [HP HQ]. split.

+ apply HQ.

+ apply HP.

Qed.

This shows why the inductive definition of and can be
manipulated by tactics as we've been doing. We can also use it to
build proofs directly, using pattern-matching. For instance:

Definition and_comm'_aux P Q (H : P ∧ Q) :=

match H with

| conj HP HQ ⇒ conj HQ HP

end.

Definition and_comm' P Q : P ∧ Q ↔ Q ∧ P :=

conj (and_comm'_aux P Q) (and_comm'_aux Q P).

#### Exercise: 2 stars, optional (conj_fact)

Construct a proof object demonstrating the following proposition.
☐
The inductive definition of disjunction uses two constructors, one
for each side of the disjunct:

## Disjunction

Module Or.

Inductive or (P Q : Prop) : Prop :=

| or_introl : P → or P Q

| or_intror : Q → or P Q.

End Or.

This declaration explains the behavior of the destruct tactic on
a disjunctive hypothesis, since the generated subgoals match the
shape of the or_introl and or_intror constructors.
Once again, we can also directly write proof objects for theorems
involving or, without resorting to tactics.

#### Exercise: 2 stars, optional (or_commut'')

Try to write down an explicit proof object for or_commut (without using Print to peek at the ones we already defined!).
☐
To give evidence for an existential quantifier, we package a
witness x together with a proof that x satisfies the property
P:

## Existential Quantification

Module Ex.

Inductive ex {A : Type} (P : A → Prop) : Prop :=

| ex_intro : ∀x : A, P x → ex P.

End Ex.

This may benefit from a little unpacking. The core definition is
for a type former ex that can be used to build propositions of
the form ex P, where P itself is a
The more familiar form ∃ x, P x desugars to an expression
involving ex:

*function*from witness values in the type A to propositions. The ex_intro constructor then offers a way of constructing evidence for ex P, given a witness x and a proof of P x.
Here's how to define an explicit proof object involving ex:

It has one constructor (so every proof of True is the same, so
being given a proof of True is not informative.)
False is equally simple — indeed, so simple it may look
syntactically wrong at first glance!

That is, False is an inductive type with

*no*constructors — i.e., no way to build evidence for it.# Programming with Tactics

*programs*using

*tactics*rather than explicit terms. Naturally, the answer is yes!

Definition add1 : nat → nat.

intro n.

Show Proof.

apply S.

Show Proof.

apply n. Defined.

Print add1.

(* ==>

add1 = fun n : nat => S n

: nat -> nat

*)

Compute add1 2.

(* ==> 3 : nat *)

Notice that we terminate the Definition with a . rather than
with := followed by a term. This tells Coq to enter
This feature is mainly useful for writing functions with dependent
types, which we won't explore much further in this book. But it
does illustrate the uniformity and orthogonality of the basic
ideas in Coq.

*proof scripting mode*to build an object of type nat → nat. Also, we terminate the proof with Defined rather than Qed; this makes the definition*transparent*so that it can be used in computation like a normally-defined function. (Qed-defined objects are opaque during computation.)# Equality

Module MyEquality.

Inductive eq {X:Type} : X → X → Prop :=

| eq_refl : ∀x, eq x x.

Notation "x = y" := (eq x y)

(at level 70, no associativity)

: type_scope.

The way to think about this definition is that, given a set X,
it defines a

*family*of propositions "x is equal to y," indexed by pairs of values (x and y) from X. There is just one way of constructing evidence for each member of this family: applying the constructor eq_refl to a type X and a value x : X yields evidence that x is equal to x.#### Exercise: 2 stars (leibniz_equality)

The inductive definition of equality corresponds to*Leibniz equality*: what we mean when we say "x and y are equal" is that every property on P that is true of x is also true of y.Lemma leibniz_equality : ∀(X : Type) (x y: X),

x = y → ∀P:X→Prop, P x → P y.

Proof.

(* FILL IN HERE *) Admitted.

☐
We can use eq_refl to construct evidence that, for example, 2 =
2. Can we also use it to construct evidence that 1 + 1 = 2?
Yes, we can. Indeed, it is the very same piece of evidence! The
reason is that Coq treats as "the same" any two terms that are

*convertible*according to a simple set of computation rules. These rules, which are similar to those used by Compute, include evaluation of function application, inlining of definitions, and simplification of matches.
The reflexivity tactic that we have used to prove equalities up
to now is essentially just short-hand for apply refl_equal.
In tactic-based proofs of equality, the conversion rules are
normally hidden in uses of simpl (either explicit or implicit in
other tactics such as reflexivity). But you can see them
directly at work in the following explicit proof objects:

Definition four' : 2 + 2 = 1 + 3 :=

eq_refl 4.

Definition singleton : ∀(X:Set) (x:X), []++[x] = x::[] :=

fun (X:Set) (x:X) ⇒ eq_refl [x].

End MyEquality.

Definition quiz6 : ∃x, x + 3 = 4

:= ex_intro (fun z ⇒ (z + 3 = 4)) 1 (refl_equal 4).

## Inversion, Again

- takes a hypothesis H whose type P is inductively defined,
and
- for each constructor C in P's definition,
- generates a new subgoal in which we assume H was
built with C,
- adds the arguments (premises) of C to the context of
the subgoal as extra hypotheses,
- matches the conclusion (result type) of C against the
current goal and calculates a set of equalities that must
hold in order for C to be applicable,
- adds these equalities to the context (and, for convenience,
rewrites them in the goal), and
- if the equalities are not satisfiable (e.g., they involve things like S n = O), immediately solves the subgoal.

- generates a new subgoal in which we assume H was
built with C,

*Example*: If we invert a hypothesis built with or, there are two constructors, so two subgoals get generated. The conclusion (result type) of the constructor (P ∨ Q) doesn't place any restrictions on the form of P or Q, so we don't get any extra equalities in the context of the subgoal.

*Example*: If we invert a hypothesis built with and, there is only one constructor, so only one subgoal gets generated. Again, the conclusion (result type) of the constructor (P ∧ Q) doesn't place any restrictions on the form of P or Q, so we don't get any extra equalities in the context of the subgoal. The constructor does have two arguments, though, and these can be seen in the context in the subgoal.

*Example*: If we invert a hypothesis built with eq, there is again only one constructor, so only one subgoal gets generated. Now, though, the form of the refl_equal constructor does give us some extra information: it tells us that the two arguments to eq must be the same! The inversion tactic adds this fact to the context.